Fuel distribution in multi-fuel tank compressed gas fuel systems

ABSTRACT

A method for distributing fuel in a fuel system of a motor vehicle. The method may be applied in a fuel system having a first fuel tank, where fuel is confined at a first pressure, and a second fuel tank, where fuel is confined at a second pressure greater than the first pressure. The method comprises releasing fuel already resident in the second fuel tank to the first fuel tank, and admitting fuel to the first and second fuel tanks simultaneously.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/477,317, filed on Jun. 3, 2009, now U.S. Pat. No. 8,443,820,the entire contents of which is incorporated herein by reference for allpurposes.

TECHNICAL FIELD

The present application relates to the field of fuel systems, and moreparticularly, to compressed-gas fuel systems for motor vehicles.

BACKGROUND AND SUMMARY

Compressed gasses such as hydrogen, methane, and natural gas (CNG) aresuitable fuels for internal combustion engines. In some localities,gaseous fuels may be less expensive than gasoline or diesel fuel interms of their cost per energy equivalent. Further, in contrast toliquid fuels, gaseous fuel may be more accurately controlled at or nearstoichiometry during an engine start, as less excess fueling is used toachieve desired combustion performance. Moreover, some gaseous fuels mayhave higher octane ratings than liquid fuels.

A challenge for gaseous fuel systems in motor vehicles is the low andtemperature-dependent volumetric energy density of compressed-gas fuels.One consequence is that the mechanical work needed to fill acompressed-gas fuel tank may be a significant fraction of the totalinternal energy stored in the fuel tank. Further, as the density of agas at constant pressure decreases with increasing temperature, the veryact of filling the fuel tank may, under some conditions, increase thetemperature of the gas such that a fuel tank filled to a constantpressure contains less fuel mass than would be present if the fuel tankwas filled to the same pressure at ambient temperature.

Various attempts to address these issues have appeared. For example,U.S. Patent Application Publication 2007/0000563 provides a system forincreasing the overall efficiency of a high-pressure gas-fueled vehicleand refilling station infrastructure. In the disclosed system, theevolved heat from high-pressure refueling is absorbed by amelting/solidifying medium inside the fuel tank, and may be dispersedvia an external radiator. In this manner, a denser charge of fuel may beadmitted to the fuel tank.

However, the approach cited above requires extensive hardware andspecial materials dedicated exclusively to temperature management. Theinventors herein have recognized this limitation and have provided amore elegant approach that may be implemented in a motor-vehicle fuelsystem having multiple compressed-gas fuel tanks Further, the system maybe integrated with a method for minimizing the mechanical work needed torefill a fuel system having multiple fuel tanks

Therefore, in one embodiment, a method for distributing fuel in a fuelsystem of a motor vehicle is provided. The method may be applied in afuel system having a first fuel tank, where fuel is confined at a firstpressure, and a second fuel tank, where fuel is confined at a secondpressure greater than the first pressure. The method comprises releasingfuel already resident in the second fuel tank to the first fuel tank,and admitting fuel to the first and second fuel tanks simultaneously.Other embodiments provide other, more particular methods fordistributing fuel in a fuel system of a motor vehicle. In this manner, amore simply configured fuel system may be operated to accommodate adenser charge of fuel than might otherwise be possible.

It will be understood that the summary above is provided to introduce insimplified form a selection of concepts that are further described inthe detailed description, which follows. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined by the claims that follow the detailed description. Further,the claimed subject matter is not limited to implementations that solveany disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows aspects of an example fuel system configuredto distribute fuel among a plurality of fuel tanks in accordance withthe present disclosure.

FIG. 2 schematically shows aspects of a second example fuel systemconfigured to distribute fuel among a plurality of fuel tanks inaccordance with the present disclosure.

FIG. 3 schematically shows aspects of a third example fuel systemconfigured to distribute fuel among a plurality of fuel tanks inaccordance with the present disclosure.

FIG. 4 illustrates an example method for distributing fuel in a fuelsystem of a motor vehicle in accordance with the present disclosure.

FIG. 5 illustrates another example method for distributing fuel in afuel system of a motor vehicle in accordance with the presentdisclosure.

FIG. 6 illustrates an example method for emulating a passive refill of afuel system of a motor vehicle in accordance with the presentdisclosure.

DETAILED DESCRIPTION

The subject matter of the present disclosure is now described by way ofexample and with reference to certain illustrated embodiments.Components that may be substantially the same in two or more embodimentsare identified coordinately and are described with minimal repetition.It will be noted, however, that components identified coordinately indifferent embodiments of the present disclosure may be at least partlydifferent. It will be further noted that the drawings included in thisdisclosure are schematic. Views of the illustrated embodiments aregenerally not drawn to scale; aspect ratios, feature size, and numbersof features may be purposely distorted to make selected features orrelationships easier to see.

FIG. 1 schematically shows aspects of an example fuel system 10configured to distribute fuel among a plurality of fuel tanks The fuelsystem may be installed in a motor vehicle. In the illustratedembodiment, pressurized, gaseous fuel is admitted to the fuel system viafuel port 12. Accordingly, the fuel port may comprise one or morefixtures and/or fittings switchably couplable to a fuel supply lineexternal to the fuel system. The fuel port may further comprise aclosure that opens to admit one or more of a fuel and the supply lineproviding the fuel.

Industry standards may establish the type of fixture or fitting (viz.,the configuration, dimensions, and/or materials thereof) through whichone or more fuels may be supplied to fuel system 10 via fuel port 12.Industry standards may further establish an acceptable range ofpressures at which one or more fuels are supplied to the fuel system.For example, hydrogen may be supplied at a pressure of approximately5000 p.s.i., or CNG may be supplied at a pressure of approximately 3600p.s.i. It will be understood that the supply pressures indicated hereinare examples only, as other suitable pressures and pressure ranges arecontemplated as well.

Continuing in FIG. 1, fuel system 10 includes check valves 14 and 16arranged in series, downstream of fuel port 12, and leading to fuel line18. The check valves prevent the pressurized fuel from escaping the fuelsystem when a closure of the fuel port is breeched. The check valves mayalso provide a predictable restriction against gas flow into the fuelsystem, such that a flow rate of fuel into the fuel system may beestimated based on a known supply pressure and a measured pressure offuel in fuel line 18. In other embodiments, a flow sensor (not shown inthe drawings) may be provided at the fuel port and configured to measurethe flow rate of fuel into the fuel system.

Fuel system 10 further includes fuel tanks 20A, 20B, 20C, and 20D,configured to store pressurized, gaseous fuel. Fluidically coupled toeach of the fuel tanks is a temperature sensor and a pressure sensor.Accordingly, temperature sensor 22A and pressure sensor 24A arefluidically coupled to fuel tank 20A, temperature sensor 22B andpressure sensor 24B are fluidically coupled to fuel tank 20B, etc. Eachtemperature sensor is responsive to the temperature of the gas insidethe fuel tank to which it is coupled, and each pressure sensor isresponsive to the pressure of a gas inside the fuel tank to which it iscoupled—either a relative pressure measured with respect to atmosphere,or an absolute pressure measured with respect to vacuum. In theembodiment illustrated in FIG. 1, the pressure sensors and temperaturesensors are operatively coupled to electronic control system 26, whichmay be any electronic control system of the fuel system or of the motorvehicle in which the fuel system is installed. Operatively coupled tothe temperature and pressure sensors as described hereinabove, theelectronic control system may be configured to estimate the amount offuel contained in each fuel tank of the fuel system.

During refueling, each fuel tank in fuel system 10 may be charged via anormally closed control valve fluidically coupled to fuel line 18.During operation of the motor vehicle, each fuel tank may be dischargedto the fuel line through the same control valve. Accordingly, controlvalve 28A is fluidically coupled to fuel tank 20A, control valve 28B isfluidically coupled to fuel tank 20B, etc. Further, control valves 28A-Dmay be operatively coupled to electronic control system 26 such thateach of the control valves may be opened, closed, and/or adjusted inresponse to a signal from the electronic control system via appropriatevalve actuators (not shown in the drawings).

From fuel line 18, fuel flows through filter 30 to pressure regulator32. The pressure regulator is configured to maintain a substantiallyconstant pressure of fuel in fuel rail 34, via which fuel is supplied tofuel injectors 36 of an engine. The filter is configured to protect thepressure regulator and fuel injectors from damage due to particulatesolids entrained in the gas stream.

The embodiment illustrated in FIG. 1 enables active charging anddischarging of fuel tanks 20A-D via the electronically controlledcontrol valves fluidically coupled to each of the fuel tanks Moreover,it enables the fuel pressures in two or more unequally filled fuel tanksto be rapidly pressure equilibrated by opening the control valvescoupled to those tanks; this feature provides certain advantages, as setforth hereinafter.

Other embodiments fully consistent with the present disclosure mayinclude more or fewer fuel tanks, more or fewer pressure sensors, an/ormore or fewer temperature sensors. In one embodiment, for example, asingle pressure sensor may be fluidically coupled to the fuel line, andthe temperature sensor may be an ambient temperature sensor. In thisembodiment, the electronic control system may be configured to estimatethe temperature of the fuel in each of the fuel tanks based on theambient temperature and on the history of fuel-line pressure variationsand valve openings/closures recorded in the electronic control system asa function of time.

FIG. 2 schematically shows aspects of a second example fuel system 38configured to distribute fuel among a plurality of fuel tanks The fuelsystem may be installed in an electric or hybrid motor vehicle, forexample. In the embodiment illustrated in FIG. 2, pressure regulator 32is coupled fluidically to an anode of fuel-cell 40, which admits air atthe cathode. The electrical output of the fuel cell is provided tostorage battery 42 and to motor 44 via power-routing network 46.

FIG. 3 schematically shows aspects of a third example fuel system 48configured to distribute fuel among a plurality of fuel tanks Instead ofa single fuel line conducting fuel from fuel port 12 and to pressureregulator 30, the embodiment illustrated in FIG. 3 includes fuel supplyline 50 fluidically coupled to the fuel port via check valves 14 and 16,and fuel delivery line 52 fluidically coupled to the pressure regulator.In this embodiment, each fuel tank is fluidically coupled to the fueldelivery line via a control valve and to the fuel supply line via acheck valve. Accordingly, check valve 54A is fluidically coupled to fueltank 20A, check valve 54B is fluidically coupled to fuel tank 20B, etc.This embodiment enables active discharging and passive (e.g., unpowered)charging of fuel tanks 20A-D. In particular, it enables any fuel tank tobe charged whenever the pressure in fuel supply line 50 is greater thanthe pressure in the fuel tank. Moreover, it enables the fuel pressuresin two or more unequally filled fuel tanks to be rapidly equilibrated byopening the control valves coupled to those tanks

The embodiments described above provide various advantages over existingmotor-vehicle fuel systems in which a check valve is integrated in eachof the control valves fluidically coupled to the fuel tanks There, theintegrated check valve may be coupled to both ends of the control valveand oriented to allow a fuel tank to fill whenever the pressure in thefuel line is greater than the pressure of the fuel tank. Fuel-tankassemblies including an integrated check valve and control valve arecommercially available. One such fuel-tank assembly 20′ is shown byexample in FIG. 1. Fuel tank assembly 20′ includes an integrated checkvalve 56 and an integrated, normally closed control valve 58 coupled inparallel to the outlet of fuel tank 60. In configurations that use sucha fuel-tank assembly, normally closed control valve 58 may beoperatively coupled to an electronic control system, such as electroniccontrol system 26. It will be noted, however, that fuel tank assembly20′ would require an additional control valve in order to allow the fueltanks to empty one at a time. The ability to empty the fuel tanks insequence, rather than simultaneously, is used to an advantage in theexample methods described hereinafter.

Nevertheless, it may be desirable—e.g., to reduce manufacturing cost—touse one or more commercially available fuel system assemblies to enactthe various fuel-distribution approaches disclosed herein. For example,it may be desirable to use fuel tank assembly 20′ in place of one ormore of the fuel tanks in fuel system 10. If used in place of fuel tank20A in fuel system 10, for instance, fuel tank assembly 20′ could bedischarged when control valves 28A and 58 are both open, and chargedwhen control valve 28A is open, but control valve 58 is closed.Likewise, if used in place of fuel tank 20A in fuel system 48, fuel tankassembly 20′ could be discharged when control valves 28A and 58 are bothopen, and charged whenever the pressure in fuel supply line 50 isgreater than the pressure in fuel tank 60.

In still other another embodiments, control valves 28A-D in fuel system10 may be normally open control valves. This variation will allow thefuel system to charge passively and discharge actively, substantially asdescribed for fuel system 48.

The configurations illustrated above enable various methods fordistributing fuel in a fuel system of a motor vehicle. Accordingly, somesuch methods are now described, by way of example, with continuedreference to above configurations. It will be understood, however, thatthese methods, and others fully within the scope of the presentdisclosure, may be enabled via other configurations as well.

FIG. 4 illustrates an example method 62 for distributing fuel—hydrogenor compressed natural gas, for example—in a fuel system of a motorvehicle. Method 62 is a fuel-system discharging method; it may beenacted when the motor vehicle is operating and when the fuel system isdelivering fuel to one or more fueled components of the motor vehicle—afuel injector or fuel cell, for example. The method ensures inter aliathat each fuel tank in the fuel system is depleted before fuel is drawnfrom the next fuel tank in sequence. In this manner, fuel-system entropyis kept as low as possible, which enables certain advantages, asdescribed hereinafter.

Method 62 and subsequent methods include various computation,comparison, and decision-making actions, which may be enacted via anelectronic control system (e.g., electronic control system 26) of thefuel system or of a motor vehicle in which the fuel system is installed.The methods further include various measuring and/or sensing actionsthat may be enacted via one or more sensors disposed in the fuel system(pressure sensors, temperature sensors, breech sensors,etc.)—operatively coupled to the electronic control system, as describedin the example configurations hereinabove. The methods further includevarious valve-actuating events, which the electronic control system mayenact in response to the various decision-making actions.

Method 62 may be entered upon when the fuel system is releasing fuelfrom one or more fuel tanks via one or more control valves (controlvalve 28A, for example) open to fuel line 18. The method begins at 64,where P_(line), the pressure of fuel in fuel line 18, is compared to aset-point pressure P_(reg) of pressure regulator 32. In embodimentswhere a pressure sensor is coupled directly to the fuel line, theelectronic control system may determine P_(line) directly. In otherconfigurations, the electronic control system may determine or estimateP_(line) indirectly. In one embodiment, P_(line) may be calculated basedon the output of a pressure sensor of a fuel tank that is open to thefuel line (one pressure sensors 24A-D, for example), and further basedon a known flow rate through fuel injectors 36.

If it is determined that P_(line) is significantly greater than P_(reg),then method 62 returns to 64, where fuel continues to be released fromthe one or more fuel tanks currently open to the fuel line 18, and wherethe pressures are compared again. However, if it is determined thatP_(line) is not significantly greater than P_(reg), then method 62advances to 66.

At 66, all currently open control valves are closed, thereby preventingsubsequent inflow of fuel to a depleted fuel tank when a fuller fueltank opens to fuel line 18. Then, at 68, the electronic control systeminterrogates the fuel pressures of each fuel tank in the fuel system,attempting to identify a fuel tank having the lowest fuel pressureP_(tank) that is greater than the set-point pressure P_(reg). In fuelsystems comprising a pressure sensor for each fuel tank, the fuelpressures may be interrogated simply by reading the response of each ofthe sensors. However, in embodiments where multiple fuel tanks share acommon pressure sensor coupled to the fuel line or fuel-delivery line(viz., the high-pressure side of regulator 32), step 68 may comprisemomentarily opening the control valves to each fuel tank in sequence toenable the common pressure sensor to sense the pressure in each fueltank one at a time.

At 70, it is determined whether a fuel tank having a pressure greaterthan or equal to the set-point pressure can be found (i.e., identified)in the fuel system. If such a fuel tank is found, then at 72, thecontrol valve linking the found fuel tank to the fuel line is open,allowing the fuel line to be sourced via the found fuel tank. In thismanner, fuel is delivered from each fuel tank to a fueled component ofthe motor vehicle only after all other fuel tanks filled to a lesserpressure are depleted. Method 62 then returns to 64. However, if it isdetermined at 70 that no fuel tank can be found having P_(tank)>P_(reg),then at 74, the electronic control system registers and indicates avacant condition of the fuel system.

The present disclosure contemplates other fuel-system dischargingmethods as well. For example, in fuel-system configurations as shown inFIG. 3, steps 64 and 66 may be omitted. In these and similarembodiments, check valves 54A-D prevent fuel from the newly opened fueltank from entering and partly refilling the emptied tanks, therebypreserving the ability to cool the fuel tanks on refill, as describedbelow.

FIG. 5 illustrates another example method 76 for distributing fuel in afuel system of a motor vehicle. Method 76 is a fuel-system refuelingmethod; it may be enacted when the motor vehicle pulls into a fillingstation or home refilling appliance and a fuel-system refill isinitiated.

Method 76 begins at 78, where the electronic control system determineswhich of at least two possible refueling modes is selected: an ECONOMYmode or a EXTENDED-FILL mode. A motor-vehicle operator orfilling-station attendant may select the refueling mode in any suitablemanner. Accordingly, the fuel system may be configured to communicatethe selected refueling mode to the electronic control system. Therefueling mode may be communicated, for example, via a fuel door keyposition or other mechanical switch. A precondition for both refuelingmodes is that the fuel tanks of the fuel system were not all dischargedsimultaneously prior to the execution of method 76, so that at least onefuel tank in the fuel system will be filled to a greater pressure thanat least one other fuel tank. This precondition may be satisfied by anymethod that discharges the fuel tanks sequentially, such as method 62,for example.

In ECONOMY mode, the fuel tanks are filled so as to minimize themechanical work needed to admit a given mass of fuel into the fuelsystem. This is accomplished by refilling the fuel tanks without firstequalizing the pressures of the fuel tanks In EXTENDED-FILL mode, thefuel tanks are filled so as to maximize the total mass of fuel that maybe added to the one or more unfull fuel tanks of the fuel system. Thismay be accomplished by precooling two or more fuel tanks by allowing gasfrom a fuller fuel tank to expand into at least one emptier fuel tank.After the expansion stage, fuel is supplied to the two or more precooledfuel tanks simultaneously. Delivering a given mass of fuel to the two ormore fuel tanks after the expansion stage requires more mechanical workand releases more heat than delivering the same mass of fuel in ECONOMYmode. After the fuel is delivered, however, the final temperature of thefuel delivered to the two or more fuel tanks will be lower than inECONOMY mode because the temperature of the two or more fuel tanksimmediately after the expansion stage may be quite low. Therefore, agreater mass of fuel may be accommodated when the fuel system is filledin EXTENDED-FILL mode rather than ECONOMY mode.

Continuing in FIG. 5, if EXTENDED-FILL mode has not been selected, thenat 80, a passive fill of the fuel system is enacted or emulated. In afuel system configured for passive refilling, such as the one shown inFIG. 3, fuel will flow into the lowest-pressure fuel tank first. Thistank will continue to fill until its pressure approaches that of one ormore other unfull fuel tanks in the fuel system. Thereafter, fuel tanksof substantially the same pressure will fill simultaneously, and so on,until all the tanks are all full or the refueling is terminated. Thissequence minimizes the mechanical work used in filling the fuel tanks,and does so absent any active control of the fuel system.

Other fuel-system configurations, such as the one shown in FIG. 1, forexample, may be directed at 80 to emulate the passive refueling sequencedescribed above. A method for emulating a passive refueling sequence isshown in FIG. 6 in one example embodiment.

Method 82 begins at 84, where the control valve coupled to thelowest-pressure fuel tank is opened. The method then advances to 86,where one or more fuel tanks are identified whose pressures are lessthan or equal to the pressure of the fuel tank being filled. Controlvalves coupled to any such fuel tanks are then opened. The method thenadvances to 88, where it is determined whether to stop the refueling.The determination at 88 may be made automatically, based on whether therefill is complete (e.g., all control valves are open, and the pressurein the fuel line is approaching the supply pressure) or in response to atermination instruction from the motor-vehicle operator or fillingstation attendant. If it is determined not to stop the refueling, thenthe method returns to 86. Otherwise, the method advances to 90, wherethe control valves coupled to all fuel tanks in the fuel system areclosed. Method 82 then returns.

Returning now to FIG. 5, if it is determined at 78 that EXTENDED-FILLmode is selected, then at 92, the electronic control system interrogatesthe fuel pressures of each fuel tank in the fuel system, attempting toidentify a fuel tank having a lower fuel pressure P_(tank) thanP_(supply), the pressure of fuel supplied to the fuel system via theexternal supply line. In some embodiments, P_(supply) may be anindustry-standard, constant value: 3600 p.s.i. for CNG, 5000 p.s.i. forhydrogen, etc. In other embodiments, P_(supply) may vary from one fueldistribution facility to the next. In these and other embodiments, thedetermination at 92 may be based on a response of a flow sensorfluidically coupled to fuel port 12. Accordingly, a positive flow intothe fuel system may be used as an indication that P_(line) is less thanP_(supply). At 94, it is determined whether such a fuel tank can befound in the fuel system. If no such fuel tank can be found, i.e., ifevery fuel tank is full, then at 96, the electronic control systemregisters and indicates a full condition of the fuel system. However, ifat least one fuel tank is found having P_(tank)<P_(supply), then theelectronic control system selects it as TANK₁. Accordingly, TANK₁ may beselected from among a plurality of unfull fuel tanks based on the fuelpressures of each unfull fuel tank in the fuel system.

If a suitable TANK₁ is found, then method 76 advances to 98, where theelectronic control system interrogates the fuel pressures of each fueltank in the fuel system, attempting to identify a fuel tank of greaterpressure than TANK₁. At 100, it is determined whether such a fuel tankcan be found in the fuel system. If no such fuel tank can be found, thenthe expansion stage referred to above may not be possible, and themethod advances to 80, as in ECONOMY MODE. However, if it is determinedthat one or more fuel tanks having higher pressure than TANK₁ areavailable, then the electronic control system selects TANK₂ from amongthem. At 102, the control valve linking TANK₂ to fuel line 18 is opened.Accordingly, TANK₂ is selected from among the plurality of fuel tanks inthe fuel system based on a fuel pressure of each fuel tank.

In other embodiments, TANK₂ may be selected based on a fuel temperatureinstead of or in addition to the fuel pressure of each fuel tank in thefuel system. In particular, TANK₂ may be chosen so that releasing fuelalready resident in TANK₂ into TANK₁ provides a cooler temperature inTANK₁ than releasing fuel already resident in any other fuel tank intoTANK₁. In still other embodiments, the determination of which fuel tankto select may be based on calculations or numerical simulations enactedin the electronic control system based on theoretical enthalpies foradiabatic expansion and/or adiabatic compression of the fuel.

Continuing in FIG. 5, method 76 then advances to 104, where the controlvalve linking TANK₁ to fuel line 18 is open. Thus, releasing fuelalready resident in TANK₂ into TANK₁ may comprise opening the controlvalves fluidically coupled to both fuel tanks, allowing fuel alreadyresident in TANK₂ to rapidly expand into both fuel tanks Further, thefuel may expand quickly enough so that very little heat is absorbed fromthe conduits and walls of the fuel tanks during the expansion, i.e., theexpansion may occur substantially adiabatically. As a result, thetemperature of the fuel in TANK₁ and TANK₂ may be significantly reducedrelative to ambient temperatures. During and/or after the rapidexpansion, fuel from fuel port 12 flows into fuel line 18 and isadmitted to both fuel tanks simultaneously. In one embodiment, fuel maybe admitted to TANK₁ and TANK₂ beginning after the expansion stage isfinished. In another embodiment, the expansion stage may occur whilefuel is being admitted to TANK₁ and TANK₂. In either case, admittingfuel simultaneously into TANK₁ and TANK₂ may comprise flowing the fuelinto TANK₁ via a control valve coupled to TANK₁ and into TANK₂ via acontrol valve coupled to TANK₂.

In other embodiments fully consistent with this disclosure, TANK₂ may beone of a plurality of fuel tanks from which fuel already resident in thefuel system is released to TANK₁ prior to admitting the fuel to TANK₁and TANK₂ simultaneously. Further, TANK₁ may be one of a plurality offuel tanks into which fuel already resident in the fuel system isreleased from the TANK₂ prior to admitting the fuel to TANK₁ and TANK₂simultaneously. Further still, the particular groups of tanks chosen inthese cases may be based on which would ultimately enable the greatestmass of fuel to be accommodated in the fuel system during a subsequentor contemporaneous refill. This, in turn, may be based on the currentfill state of each fuel tank and the predicted level of cooling for eachfuel tank during an adiabatic expansion in several possible scenarios.Accordingly, electronic control system 26 may be configured to estimatethe mass of fuel that could be accommodated under numerous possibleequilibration/refilling scenarios, based on these considerations, andthen select the scenario that accommodates the maximum total mass offuel.

Continuing in FIG. 5, method 76 advances from 104 to 80, where a passivefill is enacted or emulated, as described hereinabove. Following thisaction, the method returns. It will be understood that the methods setforth hereinabove are examples only, and that numerous other methods arewithin the spirit and scope of this disclosure. Some such methods mayhave the same or greater complexity, while others may be simpler. Forinstance, one method may comprise, during EXTENDED-FILL mode, simplyopening all of the fuel-tank control valves simultaneously to equalizethe fuel pressure just before the refill event. After the fuel-tankpressures are equalized, refilling may be enacted passively, forexample. In yet another example, a staged refill may be enacted, whereintwo or more fuel tanks in a first group are chosen as described abovefor a first stage of pressure equalization, then after pressureequalization and a subsequent partial refueling, a second group may bechosen for a second stage of pressure equalization and continuedrefueling. In some embodiments, the second stage may be enacted as soonas the first stage is complete, while in other embodiments, the firstand second stages may be separated by any suitable period of time.

It will be further understood that the example control and estimationroutines disclosed herein may be used with various systemconfigurations. These routines may represent one or more differentprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, the disclosedprocess steps (operations, functions, and/or acts) may represent code tobe programmed into computer readable storage medium in an electroniccontrol system. It will be understood that some of the process stepsdescribed and/or illustrated herein may in some embodiments be omittedwithout departing from the scope of this disclosure. Likewise, theindicated sequence of the process steps may not always be required toachieve the intended results, but is provided for ease of illustrationand description. One or more of the illustrated actions, functions, oroperations may be performed repeatedly, depending on the particularstrategy being used.

Finally, it will be understood that the articles, systems and methodsdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are contemplated. Accordingly, the presentdisclosure includes all novel and non-obvious combinations andsub-combinations of the various systems and methods disclosed herein, aswell as any and all equivalents thereof.

1. A method for distributing fuel in a vehicle fuel system, the fuelsystem having a first fuel tank, where fuel is confined at a firstpressure, and a second fuel tank, where fuel is confined at a secondpressure greater than the first pressure, comprising: releasing naturalgas fuel already resident in the second fuel tank to the first fueltank; and admitting natural gas fuel to the first and second fuel tankssimultaneously.
 2. The method of claim 1, wherein admitting natural gasfuel to the first and second fuel tanks simultaneously begins after saidreleasing is finished.
 3. The method of claim 1, wherein said releasingoccurs while admitting natural gas fuel to the first and second fueltanks simultaneously.
 4. The method of claim 1, where said releasing andsaid admitting occur during a first refueling mode of the fuel system,the method further comprising, during a second refueling mode of thefuel system, admitting natural gas fuel to the first fuel tank while nonatural gas fuel is admitted to the second fuel tank.
 5. The method ofclaim 1, further comprising delivering natural gas fuel from the firstfuel tank to a fueled component of the motor vehicle, and, deliveringnatural gas fuel from the second fuel tank to a fueled component of themotor vehicle after the fuel in the first fuel tank is depleted.
 6. Themethod of claim 5, wherein the fueled component of the motor vehicle isa fuel injector disposed in an engine of the motor vehicle.
 7. Themethod of claim 5, wherein the fueled component of the motor vehicle isa fuel cell.
 8. The method of claim 1, wherein the second fuel tank isone of a plurality of fuel tanks from which natural gas fuel alreadyresident in the fuel system is released to the first fuel tank prior toadmitting the natural gas fuel to the first and second fuel tankssimultaneously.
 9. The method of claim 1, wherein the first fuel tank isone of a plurality of fuel tanks to which the natural gas fuel alreadyresident in the fuel system is released from the second fuel tank priorto admitting the natural gas fuel to the first and second fuel tankssimultaneously.
 10. The method of claim 1, wherein a fuel line of themotor vehicle is fluidically coupled to the first fuel tank via a firstcontrol valve and to the second fuel tank via a second control valve,and where releasing natural gas fuel already resident in the second fueltank to the first fuel tank comprises opening the first and secondcontrol valves.
 11. The method of claim 10, wherein admitting naturalgas fuel to the first and second fuel tanks simultaneously comprisesflowing the natural gas fuel into the first fuel tank via the firstcontrol valve and into the second fuel tank via the second controlvalve.
 12. The method of claim 11, further comprising closing at leastthe first control valve when a pressure of natural gas fuel in the fuelline approaches a supply pressure of the fuel.
 13. The method of claim1, wherein releasing natural gas fuel already resident in the secondfuel tank to the first fuel tank is enacted substantially adiabatically.14. A method for refueling a fuel system of a motor vehicle, the fuelsystem having a plurality of fuel tanks, comprising: selecting an unfullfirst fuel tank from among the plurality of fuel tanks; during a firstrefueling mode of the fuel system, admitting fuel to the unfull firstfuel tank only; during a second refueling mode of the fuel system:selecting from among the plurality of fuel tanks a second fuel tankhaving a greater pressure than the unfull first fuel tank; releasingfuel already resident in the second fuel tank to the unfull first fueltank; and admitting fuel to the second and unfull first fuel tankssimultaneously, where pressurized, gaseous fuel is admitted to the fuelsystem via a fuel port coupled to the plurality of fuel tanks
 15. Themethod of claim 14, wherein the unfull first fuel tank is among aplurality of unfull fuel tanks selected in sequence based on the fuelpressures of each unfull fuel tank in the plurality of unfull fuel tanks16. The method of claim 14, wherein the second fuel tank is selectedfrom among the plurality of fuel tanks based on at least one of a fuelpressure and a fuel temperature of each fuel tank in the plurality offuel tanks
 17. The method of claim 16, wherein releasing fuel alreadyresident in the second fuel tank to the unfull first fuel tank providesa cooler temperature in the unfull first fuel tank than releasing fuelalready resident in any other fuel tank in the plurality of fuel tanksto the unfull first fuel tank.
 18. The method of claim 13, wherein thefuel comprises compressed natural gas.
 19. The method of claim 13,wherein the fuel comprises hydrogen.
 20. A method for refueling a fuelsystem of a motor vehicle with compressed natural gas, the fuel systemhaving a plurality of fuel tanks, the method comprising: selecting anunfull first fuel tank from among the plurality of fuel tanks; during afirst refueling mode of the fuel system, opening a first control valveto admit compressed natural gas via a fill port to the unfull first fueltank only; during a second refueling mode of the fuel system: selectingfrom among the plurality of fuel tanks a second fuel tank having apressure of compressed natural gas greater than that of the unfull firstfuel tank; opening the first control valve and a second control valve torelease compressed natural gas already resident in the second fuel tankto the unfull first fuel tank; and admitting fuel to the second andunfull first fuel tanks via the fill port simultaneously.